1. Field of the Invention
The present invention relates to a display device for use in displaying an image by using a display means such as a plasma display, a liquid-crystal display or the like. More particularly, this invention relates to a display device in which the whole of an image may be made easy to see by increasing an intermediate luminance level while its maximum output is held at the constant level when an average luminance level of a displayed video signal is low.
2. Description of the Related Art
Heretofore, in display devices using a display means such as a plasma display and a liquid-crystal display, a contrast of a display image is lowered when a viewer watches the displayed image in the bright circumstances, for example. In particular, when a dark scene in which an object luminance level is low is displayed, a displayed image becomes difficult to see. Contrary to this device, display devices using, for example, a cathode-ray tube have hitherto been adapted to automatically increase a contrast of a displayed image (increase an amplitude of a video signal) when a study of measured results of an average luminance level of a video signal or the like reveals that its average luminance level is lowered.
In the display devices using the plasma display or the liquid-crystal display, for example, since a dynamic range of a luminance signal of these display devices is narrow, in order to effectively utilize this narrow dynamic range, it is customary that a margin of contrast is almost removed so that the maximum value of the amplitude of the ordinary video signal is already set to the limit of the dynamic range. There is then the risk that when a contrast is increased more than that, a white peal, for example, will be over the dynamic range and will be saturated.
In a scene in which only one portion is bright in the whole of a dark object, for example, when the whole contrast is increased based on the measured results of the average luminance level, there is then the risk that the signal level of the bright portion will be over the dynamic range. As a consequence, in such scene, the video signal of the above-mentioned bright portion will be saturated to cause a so-called white compression. There is then the problem that a gradation of this bright portion cannot be expressed.
The problems that the present invention intends to solve are that an image becomes difficult to see in the scene in which the object luminance level is low and that when the whole contrast is increased, in the scene in which only one portion is bright in the whole of the dark object, for example, this bright portion becomes the so-called white compression so that the gradation of this portion cannot be expressed.
In view of the aforesaid aspect, it is an object of the present invention to provide a display device in which an average luminance signal is measured from a luminance signal or three primary color signals or digital three primary color signals and a predetermined gamma correction, controlled by the average luminance signal, is carried out so that, while a maximum output of a video signal is held at a constant level, an intermediate luminance level is increased in a scene in which an object luminance level is low to thereby make an image of a dark scene become easy to see.
According to the present invention, there is provided a display device which is comprised of a separating circuit for separating a video signal inputted in the form of an analog signal to provide a luminance signal and color-difference signals or video signal input terminals to which the luminance signal and the color-difference signals are supplied separately, a converting circuit for converting the luminance signal and the color-difference signals into three primary color signals or primary signal input terminals to which the three primary color signals are supplied independently, and an A/D-converting means for converting the three primary color signals in the form of analog signals to digital signals or digital input terminals to which the thus digitally-converted three primary color signals are supplied independently.
To attain the above-mentioned object, the display device according to the present invention comprises a means for measuring an average luminance level from the luminance signal or the three primary color signals or the digital three primary color signals, and a correction means having a predetermined gamma correction curve controlled by an output signal from the measuring means.
Since the gamma correction curve of the correction means is feedforward-controlled or feedback-controlled in response to the average luminance level from the measuring means, in a scene in which an object luminance level is low, an intermediate luminance level is increased to make an image of a dark scene become easy to see. In that case, since the maximum output of the video signal is held at the constant level, even in a scene in which only one portion is bright in the whole dark object, a gradation of the bright portion may be expressed satisfactorily.
According to a first aspect of the present invention, there is provided a display device including a separating circuit for separating a video signal inputted in the form of an analog signal to provide a luminance signal and color-difference signals or video signal input terminals to which the luminance signal and the color-difference signals are supplied separately, a converting circuit for converting the luminance signal and the color-difference signals into three primary color signals or primary color signal input terminals to which the three primary color signals are supplied independently, and an A/D converting means for converting the three primary color signals in the form of analog signals to digital signals or digital input terminals to which the digitally-converted three primary color signals are supplied independently. This display device comprises a measuring means for measuring an average luminance level of the luminance signal supplied from the separating circuit or video signal input terminal or the three primary color signals supplied from the converting circuit or the primary color signal input terminals or the digitally-converted three primary color signals supplied from the A/D converting means or digital input terminals, and a gamma correction means having a gamma correction curve controlled by an output control signal from the measuring means.
According to a second aspect of the present invention, in the above-mentioned display device, the measuring means includes a comparing means for being supplied with an analog luminance signal from the separating circuit or the video signal input terminals and for comparing the analog luminance signal with a predetermined level, an integrating circuit for integrating an output from the comparing circuit, and a control circuit for generating an output control signal based on an integrated value from the integrating circuit.
According to a third aspect of the present invention, in the above-mentioned display device, the measuring means includes an analog luminance signal generating circuit for being supplied with analog three primary color signals from the converting circuit or the primary color signal input terminals and for generating an analog luminance signal by adding the three primary color signals with a predetermined ratio, a comparing circuit for comparing the generated analog luminance signal with a predetermined level, an integrating circuit for integrating an output from the comparing circuit, and a control circuit for generating an output control signal based on an integrated value from the integrating circuit.
According to a fourth aspect of the present invention, in the above-mentioned display device, the measuring means includes a digital luminance signal generating circuit for being supplied with the digitally-converted three primary color signals from the A/D converting means or the digital input terminals and for generating a digital luminance signal by adding the digitally-converted three primary color signals with a predetermined ratio, a comparing circuit for comparing the generated digital luminance signal with a predetermined level, an integrating circuit for integrating an output from the integrating circuit, and a control circuit for generating an output control signal based on an integrated value from the integrating circuit.
According to a fifth aspect of the present invention, in the above-mentioned display device, the gamma correction curve of the gamma correction means has a control characteristic such that the gamma correction curve is made approximately a straight line when the level of the output control signal from the measuring means is large and that an intermediate signal level is increased as the level of the output control signal from the measuring means is lowered.
According to a sixth aspect of the present invention, in the above-mentioned display device, the gamma correction means is provided with respect to the analog luminance signal supplied from the separating circuit or the video signal input terminals, the output control signal from the measuring means is generated based on the analog luminance signal from the separating circuit or the video signal input terminals and the gamma correction means is feedforward-controlled by the output control signal from the measuring means.
According to a seventh aspect of the present invention, in the above-mentioned display device, the gamma correction means is provided with respect to the analog luminance signal supplied from the separating circuit or the video signal input terminals, the output control signal from the measuring means is generated based on the analog luminance signal outputted from the gamma correction means and the gamma correction means is feedback-controlled by the output control signal from the measuring means.
According to an eight aspect of the present invention, in the above-mentioned display device, the gamma correction means is provided with respect to the analog luminance signal supplied from the separating circuit or the video signal input terminals, the output control signal from the measuring means is generated based on the analog three primary color signals outputted from the converting circuit and the gamma correction means is feedback-controlled by the output control signal from the measuring means.
According to a ninth aspect of the present invention, in the above-mentioned display device, the gamma correction means is provided with respect to the analog luminance signal from the separating circuit or the video signal input terminals, the output control signal supplied from the measuring means is generated based on the digitally-converted three primary color signals outputted from an A/D converting means and the gamma correction means is feedback-controlled by the output control signal from the measuring means.
According to a tenth aspect of the present invention, in the above-mentioned display device, the gamma correction means is provided with respect to the analog luminance signal supplied from the separating circuit or the video signal input terminals, and also provided with a color gain control means in order to control levels of two color-difference signals comprising the video signal in response to the measured average luminance level, the output control signal from the measuring means is generated based on the analog luminance signal supplied from the separating circuit or the video signal input terminals, and the gamma correction means and the color gain control means are feedforward-controlled by the output control signal from the measuring means.
According to an eleventh aspect of the present invention, in the above-mentioned display device, the gamma correction means is provided with respect to the analog luminance signal supplied from the separating circuit or the video signal input terminals, also provided with a color gain control means in order to control levels of two color-difference signals comprising the video signal in response to the measured average luminance level, the output control signal from the measuring means is generated based on the analog luminance signal supplied from the gamma correction means, and the gamma correction means and the color gain control means are feedback-controlled by the output control signal from the measuring means.
According to a twelfth aspect of the present invention, in the above-mentioned display device, the gamma correction means is provided with respect to the analog luminance signal supplied from the separating circuit or the video signal input terminals, also provided with a color gain control means in order to control levels of two color-difference signals comprising the video signal in response to the measured average luminance level, the output control signal from the measuring means is generated based on the analog three primary color signals outputted from the converting means and the gamma correction means and the color gain control means are feedback-controlled by the output control signal from the measuring means.
According to a thirteenth aspect of the present invention, in the above-mentioned display device, the gamma correction means is provided with respect to the analog luminance signal supplied from the separating circuit or the video signal input terminals, also provided with a color gain control means in order to control levels of two color-difference signals comprising the video signal in response to the measured average luminance level, the output control signal from the measuring means is generated based on the digitally-converted three primary color signals outputted from the A/D converting means, and the gamma correction means and the color gain control means are feedback-controlled by the output control signal from the measuring means.
According to a fourteenth aspect of the present invention, in the above-mentioned display device, the gamma correction means are provided with respect to respective three primary color signals outputted from the converting circuit, the output control signal from the measuring means is generated based on the analog luminance signal supplied from the separating circuit or the video signal input terminals, and the gamma correction means are respectively feedforward-controlled by the output control signal from the measuring means.
According to a fifteenth aspect of the present invention, in the above-mentioned display device, the gamma correction means are provided with respect to the respective three primary color signals outputted from the converting circuit or the primary color signal input terminals, the output control signal from the measuring means is generated based on the analog three primary color signals supplied from the converting circuit or the primary color signal input terminals, and the gamma correction means are respectively feedforward-controlled by the output control signal from the measuring means.
According to a sixteenth aspect of the present invention, in the above-mentioned display device, the gamma correction means are provided with respect to the respective three primary color signals supplied from the converting circuit or the primary color signal input terminals, the output control signal from the measuring means is generated based on the analog three primary color signals outputted from the gamma correction means, and the gamma correction means are respectively feedback-controlled by the output control signal from the measuring means.
According to a seventeenth aspect of the present invention, in the above-mentioned display device, the gamma correction means are provided with respect to the respective three primary color signals supplied from the converting circuit or the primary color signal input terminals, the output control signal from the measuring means is generated based on the digitally-converted three primary color signals outputted from the A/D converting means, and the gamma correction means are respectively feedback-controlled by the output control signal from the measuring means.
According to an eighteenth aspect of the present invention, in the above-mentioned display device, the gamma correction means are provided with respect to the respective digitally-converted three primary color signals outputted from the A/D converting means, the output control signal from the measuring means is generated based on the analog luminance signal supplied from the separating circuit or the video signal input terminals, and the gamma correction means are respectively feedforward-controlled by the output control signal from the measuring means.
According to a nineteenth aspect of the present invention, in the above-mentioned display device, the gamma correction means are provided with respect to the respective digitally-converted three primary color signals outputted from the A/D converting means, the output control signal from the measuring means is generated based on the analog three primary color signals supplied from the converting circuit or the primary color input terminals, and the gamma correction means are respectively feedforward-controlled by the output control signal from the measuring means.
According to a twentieth aspect of the present invention, in the above-mentioned display device, the gamma correction means are provided with respect to the respective digitally-converted three primary color signals supplied from the A/D converting means or the digital input terminals, the output control signal from the measuring means is generated based on the digitally-converted three primary color signals supplied from the A/D converting means or the digital input terminals, and the gamma correction means are respectively feedforward-controlled by the output control signal from the measuring means.
In accordance with a twenty-first aspect of the present invention, in the above-mentioned display device, the gamma correction means are provided with respect to the respective three primary color signals supplied from the A/D converting means or the digital input terminals, the output control signal from the measuring means is generated based on the digital three primary color signals outputted from the gamma correction means, and the gamma correction means are respectively feedback-controlled by the output control signal from the measuring means.